Right at the beginning of the crisis (and before it had hit the UK) I remember talking to someone whose parents live in China. I forget all the finer detail, but they were allocated a coloured card, based on the first letter of their surname. It was linked to their car, and I think there was one card per household. People were allowed out on a rota system, based on their colour, and could be asked to show their card at any time.
This is obviously draconian, but so is basing it on age, and in some ways I think it is a better idea. It would mean that mixed age couples could go out together, siblings could potentially visit older parents at different times, and more workplaces could stay open based on shiftwork. It also means that if there were an outbreak, it is likely to be confined to one 'colour', who could be more easily isolated.